1. Field of the Invention
The present invention relates to an image sensor and more particularly to a MOS (Metal Oxide Semiconductor) type image sensor in which the dispersion in a threshold voltage of a transistor constituting a source follower to output a photoelectric conversion voltage of a pixel to the outside is compensated.
2. Description of the Related Art
An image sensor is a sensor to convert optical image information to an electrical signal in a TV camera and the like. A MOS-type image sensor in particular is so configured as to have a photodiode as a photoelectric conversion device and its peripheral devices composed of a MOS-type FET (Field Effect Transistor), which is featured by low power consumption, low costs, etc.
FIG. 5 is a block diagram illustrating configurations of a conventional MOS-type image sensor. Configurations and operations of the MOS-type image sensor are hereafter described referring to FIG. 5.
As depicted in FIG. 5, a conventional MOS-type image sensor is composed of a pixel array 31 in which many unit pixels (i.e., picture elements) 30 are arrayed in a matrix-like manner in horizontal (low) and vertical (column) directions, an address decoder 32, a vertical shift register 33, a vertical driver 34, a clock control circuit 35, a noise control circuit 36 and a horizontal shift register 37.
The unit pixel 30 consists of a photodiode 301 being a photoelectric conversion device, a transistor 302 to reset the photodiode to a power supply voltage Vcc, a transistor to amplify a photoelectric conversion voltage of the photodiode 301 and a read-out transistor to connect a transistor 303 for amplification to a bit line BL corresponding with activation of a word line WL.
An address signal read out from a control circuit not shown is divided, via the address decoder 32, into two signals, one going in the vertical direction and the other in the horizontal direction. The vertical shift register 33, when receiving the address signal in the vertical direction, generates a read-out control signal which is shifted sequentially in the vertical direction. The control signal is sequentially fed to each word line WL so that a pixel in the pixel array 31 for each word line can be read out. On the other hand, the horizontal shift register 37, when receiving the address signal in the horizontal direction, selects sequentially the bit line BL to connect it to the noise control circuit 36 and reads out the photoelectric conversion voltage of the photodiode 301 for every unit pixel at a cross point of the word line WL and the bit line BL and then, after noise control, generates an image signal output. At this point, the noise control circuit 36 is adapted to remove noises caused by switching operations of each gate transistors which are produced in a manner overlapping the photoelectric conversion output fed from the photodiode 301.
There are conventionally a variety of types of circuits for reading outputs and removing noises in such a MOS-type image sensor.
FIG. 6 is a block diagram illustrating a conventional image sensor and FIG. 7 is a timing chart showing operations of the circuit shown in FIG. 7.
As shown in FIG. 6, the conventional image sensor is composed of unit pixels 41 and 42 located on an arbitrary n-th (n=0,1,2 . . . ) line and in a n-th+1st string being neighboring to an arbitrary n-th string in a pixel array constituting the image sensor, noise control circuits 43 and 44 on the n-th string and n-th+1st string respectively, a current source 45 constituting a source follower together with transistors 203 of the noise control circuit 43 and the like, a P channel transistor 46 constituting the source follower, a current source 47 and a P channel transistor 48. The number of lines and strings constituting the pixel array is configured arbitrarily.
The pixel 41 consists of a photodiode 101 being a photoelectric conversion device, N channel transistors 102, 103 and 104, and a current source 105.
The photodiode 101 is adapted to convert incident light at the unit pixel to an electrical signal. The transistor 102 resets an initial voltage of the photodiode 101 to a power supply voltage Vcc in accordance with a reset control signal RSTn on the n-th line. The transistor 103 constitutes a source follower to amplify the photoelectric conversion voltage of the photodiode 101 together with the current source 105. The transistor 104 serves to connect the transistor 103 through a data output line DATAn on the n-th string to the current source 105 in accordance with a word line read-out control signal WLn on the n-th line. The current source 105 feeds a constant current to the transistor 103 connected through a gate composed of a transistor 104. Configurations and functions of a pixel 42 are the same as the pixel 41.
The noise control circuit 43 is composed of N channel transistors 201, 203 and 204, a coupling capacitor 202 and a P channel transistor 205.
The transistor 201 connects the data output line DATAn to the coupling capacitor 202 in accordance with a signal voltage read-out control signal SHS. The coupling capacitor acts to transmit a change of an output voltage of the data output line DATAn to a node S/Hn. The transistor 203 constitutes a source follower together with the current source 45 and outputs a voltage of the node S/Hn to a transistor 46. The transistor 204 connects the transistor 203 to the current source 45 in accordance with a bit line read-out control signal YSWn. The transistor 205 connects the node S/Hn to a power supply source OCVn in accordance with a clamp control signal OCI. Configurations and functions of a noise control circuit 44 and other noise control circuits not shown are the same as the noise control circuit 43.
A current source 45 feeds a constant current to the transistors 203 and the like. The transistor 46 constitutes a source follower together with a current source 47 and outputs an output voltage Vout in accordance with a voltage of a gate. The transistor 48 connects the transistor 46 to the current source 47 in accordance with an output enable signal OE.
Operations of the conventional image sensor shown in FIG. 6 are described by referring to FIG. 7. Operations of the pixel 41 and the noise control circuit 43 are described primarily, however, operations of the pixel 42 and other pixels not shown and of the noise control circuit 44 and other noise control circuits not shown are the same as above. By making LOW a vertical read-out control signal IRASB, a vertical address of the pixel array is sequentially designated. Moreover, by making HIGH a signal voltage read-out control signal SHS, a gate composed of the transistor 201 is turned ON, causing the data output line DATAn to be connected to the coupling capacitor 202. Then, in accordance with the designated address, the word line read-out control signal WLn is made HIGH and a clamp control signal OCI is made LOW.
When a reset control signal RSTn is made HIGH by a previous resetting operation and the gate composed of the transistor 102 is turned ON, if the photodiode 101 is exposed to light for a definite time while it is charged to the power supply voltage Vcc, its voltage drops responsive to an incident light level. At this point, since the word read-out control signal WLN is made HIGH, a gate composed of the transistor 104 is turned ON, causing a photoelectric conversion voltage generated when the photodiode is exposed to light to be outputted to the data output line DATAn through a source follower composed of the transistor 103.
In this state, because the signal voltage read-out control signal SHS is made HIGH and a gate composed of the transistor 201 is turned ON, a voltage of the data output line DATAn is applied to the coupling capacitor 202. At this point, since the clamping control signal OCI is made LOW, a gate composed of the transistor 205 is turned ON and, at the same time, because the voltage of the power supply OCVn has been dropped to a voltage V1, the node S/Hn is clamped on the voltage V1, and then a voltage of the data output line DATAn is read out by the coupling capacitor 202.
Furthermore, by making LOW the signal voltage read-out control signal SHS and turning ON a gate composed of the transistor 201 temporarily, and by making HIGH a clamping control signal OCI, a gate composed of the transistor 205 is turned OFF, causing the voltage of the power supply OCVn to be restored to the power supply voltage Vcc.
Then, by making HIGH a reset control signal RSTn, a gate composed of the transistor 102 is turned ON, causing the photodiode 101 to be charged to the power supply voltage Vcc. Then, after the reset control signal RSTn is turned ON and a gate composed of the transistor 102 is turned OFF, by again making HIGH the voltage read-out control signal SHS and by turning ON the gate composed of the transistor 201, a photoelectric conversion voltage generated when the photodiode is unexposed to light is applied to the coupling capacitor. By this, an output represented by the following formula (1) is read out at the node S/Hn through the coupling capacitor 202:
V1+(photoelectric voltage generated at unexposed time)xe2x88x92(photoelectric voltage generated at exposed time)xe2x80x83xe2x80x83(1)
At this point, by making HIGH the bit line read-out control signal YSWn, a gate composed of the transistor 204 is turned ON, and a voltage of the node S/Hn is outputted at the gate of the transistor 46 through a source follower composed of the transistor 203. In this state, because the output enable signal OE goes LOW, a gate composed of the transistor 48 is turned ON, causing an output voltage Vout to be outputted through a source follower composed of the transistor 46.
Because the output voltage Vout corresponds with the remainder of the photoelectric voltage generated at the unexposed time and that generated at the exposed time as shown in the above formula (1), in the external circuits now shown, by this output voltage Vout, signal voltages from which a noise caused by switching operations of each transistor constituting the gate of the pixel 41 and the noise control circuit 43 has been removed or in which the dispersion in threshold values of the transistor 103 constituting the source follower of the pixel 11 has been offset and removed can be obtained.
However, in the above conventional image sensor, even if subtraction between the photoelectric conversion voltage generated when the pixel is unexposed to light and that generated when exposed to light is performed, the dispersion in threshold voltages of the transistor 203 constituting the source follower for coupling outputs, for example, in the noise control circuit 43 cannot be offset and removed, presenting a problem in that the output signal Vout is affected by the dispersion in threshold voltages of the transistor 202.
In view of the above, it is an object of the present invention to provide an image sensor wherein an output voltage of each column is not affected by the dispersion in a threshold voltage of a transistor constituting a source follower to output a signal output of each column to the outside.
According to a first aspect of the present invention, there is provided an image sensor comprising:
a coupling capacitor to which a photoelectric conversion voltage generated when a pixel is exposed to light and a photoelectric conversion voltage generated when said pixel is unexposed to light are sequentially applied; and
a source follower through which an output, generated at a node on the output side of said coupling capacitor, being the remainder of the photoelectric conversion voltage generated when the pixel is unexposed to light and that generated when the pixel is exposed to light is taken;
whereby the node on the output side of the capacitor is clamped on a sum of a definite voltage and a threshold voltage of a transistor constituting the source follower and, after the photoelectric conversion voltage generated when the pixel is exposed to light has been read out, the photoelectric conversion voltage generated when the pixel is unexposed to light is read out.
According to a second aspect of the present invention, there is provided an image sensor comprising:
a coupling capacitor to which a photoelectric conversion voltage generated when a pixel is exposed to light and that generated when the pixel is unexposed to light are sequentially applied; and
a source follower through which an output, generated at a node on the output side of the coupling capacitor, being the remainder of the photoelectric conversion voltage generated when the pixel is unexposed to light and that generated when exposed to light is taken;
whereby a drain of a transistor constituting the source follower, when the photoelectric conversion voltage generated when the pixel is exposed to light is read out, is clamped on a definite voltage, and a gate and a source of the above transistor is connected via gate means and, when the photoelectric conversion voltage generated when the pixel is unexposed to light is read out, the gate means is turned off and simultaneously the clamping of the drain is released.
According to a third aspect of the present invention, there is provided an image sensor comprising:
a coupling capacitor connected to a data output line for every column through a first gate means which is turned on when a signal voltage of the data output line is read out; and
a source follower used to read out a voltage at a node on the output side of the coupling capacitor;
whereby a second gate connected between a drain of a transistor constituting the source follower and the node is turned on, causing the above node to be pre-charged to a power supply voltage and then a third gate means connected between a gate of the transistor and a source is turned on and, with a voltage of a drain of the transistor dropped to a definite level, the first gate means is turned on, causing an output voltage of the data output line generated when the pixel is exposed to light to be applied to the coupling capacitor and, then, with the voltage of a drain of the transistor restored to a power supply voltage, the first gate means is again turned on, causing the output voltage of the data output line generated when the pixel is unexposed to light to be applied to the coupling capacitor.
In the foregoing, a preferable mode is one wherein the pixel is provided with a photodiode with its cathode grounded and with its anode reset to a predetermined voltage at the time of a start of operations, and with a source follower used to output a voltage of the anode of the photodiode to the data output line when data is read out.
According to a fourth aspect of the present invention, there is provided an image sensor comprising:
a coupling capacitor to which a photoelectric conversion voltage generated when a pixel is exposed to light and that generated when the pixel is unexposed to light are sequentially applied; and
a source follower through which an output, generated at a node on the output side of the coupling capacitor, being the remainder of the photoelectric conversion voltage generated when the pixel is unexposed to light and that generated when is exposed to light is taken;
whereby a node on the output side of the coupling capacitor is clamped on a sum of a definite voltage and a threshold voltage of the transistor constituting the source follower and, after the photoelectric conversion voltage generated when the pixel is unexposed to light has been read out, the photoelectric conversion voltage generated when the pixel is exposed to light is read out.
According to a fifth aspect of the present invention, there is provided an image sensor comprising:
a coupling capacitor to which a photoelectric conversion voltage generated when a pixel is exposed to light and that generated when the pixel is unexposed to light are sequentially applied; and
a source follower through which an output, generated at a node on the output side of the coupling capacitor, being the remainder of the photoelectric conversion voltage generated when the pixel is unexposed to light and that generated when the pixel is exposed to light is taken;
whereby, at the time when the photoelectric conversion voltage generated when the pixel is unexposed to light is read out, with the photodiode isolated from the pixel, the voltage of a drain of the transistor constituting the source follower is dropped to a definite level and, at the same time, a gate and a source of the transistor are connected via a gate means, and at the time when the photoelectric conversion voltage generated when the pixel is exposed to light is read out, the photodiode is connected to the pixel and the gate means is turned off and simultaneously the voltage of the drain is restored to its original level.
According to a sixth aspect of the present invention, there is provided an image sensor comprising:
a coupling capacitor connected to a data output line for every column through a first gate means which is turned on when a signal voltage of said data output line is read out; and
a source follower used to read out a voltage at a node on the output side of the coupling capacitor;
whereby, with a photodiode isolated from a pixel, a second gate between a gate and a source of a transistor constituting the source follower and, at the same time, with a drain of the transistor dropped to a definite voltage, the first gate means is turned on, causing an output voltage of a data output line in the pixel to be applied to the coupling capacitor, and then the photodiode is connected to the pixel and, with the voltage of the drain of the transistor restored to a power supply voltage, the first gate means is turned on, causing the output voltage in the data output line generated when the pixel is exposed to light to be applied to the coupling capacitor.
In the foregoing, a preferable mode is one wherein the pixel is provided with a photodiode with its cathode grounded and with its anode reset to a definite voltage at the time of a start of operations, a source follower used to output the voltage of the anode of said photodiode to the data output line when data is read out and a fourth gate means used to isolate the photodiode from aid pixel.